The 90/10 Principle states that only 10% of life is made up of external events outside of our control, while 90% is determined by our reactions to what happens. The document provides an example of how a negative reaction to spilled coffee can ruin one's whole day, while maintaining a positive reaction would have led to a much better outcome. It encourages applying this principle by not letting negative comments or delays control one's emotions and stress levels, but instead using that time and energy for more constructive purposes. Mastering this 90/10 principle, the document argues, can significantly improve one's life experience.